Chapter 10: Sexual Reproduction and Genetics 10.1 - Meiosis

Objectives Main Idea: Meiosis produces haploid gametes. Essential Questions: How does reduction in chromosome number occur during meiosis? What are the stages of meiosis? What is the importance of meiosis in providing genetic variation?

Introduction There are no two individuals that are identical to each other. Genetic variation – a variety of traits in organisms

Chromosomes and Chromosome Number Trait – Characteristics (hair color, height, or eye color) Encoded in your genetic code, DNA Genes – DNA segment that control the production of a protein.

Homologous chromosomes

Homologous Chromosomes Humans have 46 chromosomes. Each parent contributes 23 chromosomes, resulting in 23 pairs of chromosomes. Homologous Chromosomes – Chromosomes that make up a pair. One chromosome from each parent.

Haploid and Diploid Cells Gametes – sex cells that have half the number of chromosomes Haploid Cell – A cell with n number of chromosomes. Fertilization – The process by which one haploid gamete combines with another haploid gamete. Diploid Cell – A cell with 2n number of chromosomes.

Meiosis I & II

Overview Meiosis – A type of cell division that reduces the number of chromosomes. (reduction division) Meiosis will create cells that have n number of chromosomes. Meiosis starts with diploid cells which then divide and create haploid cells.

Meiosis I Interphase: Chromosomes replicate Chromatin condenses

Meiosis I Prophase I: Pairing of homologous chromosomes occurs, each chromosome consists of two chromatids. Centrioles move to opposite poles, spindle fibers form and bind to sister chromatids at centromere. The nuclear envelope breaks down. Synapsis forms between the pairs of homologous chromosomes. Chromsomes are physically bound together.

Meiosis I Prophase I: Crossing over produces exchange of genetic information. (Figure 10.4)

Prophase I

Meiosis I

Metaphase: Meiosis I vs Mitosis Metaphase in Meiosis I Metaphase in Mitosis Mitosis: sister chromatids line up at the cell’s equator Meiosis: homologous chromosomes line up at the cell’s equator

Meiosis I

Anaphase I Chromosome number is reduced from 2n to n when the homologous chromosomes separate. (Watch the Amoeba sister video on the website on demystifying chromosome reduction.)

Telophase I

Meiosis II Meiosis is only halfway completed at the end of meiosis I. Meiosis II will continue to divide the chromosomes to produce haploid cells.

Prophase II Chromosomes condense. Spindles from in each new cell. Spindle fibers attach to chromosomes.

Metaphase II Centromeres of chromosomes line up randomly at the equator of each cell. Difference between Mitosis and Meiosis: Mitosis: a diploid number of chromosomes line up at the equator Meiosis: a haploid number of chromosomes line up at the equator

Anaphase II Centromeres split. Sister chromatids separate and move to opposite poles.

Telophase II Chromosomes reach the poles. Four nuclei form around chromosomes. Spindles break down. Cells divide.

Product Four cells have formed. Each nucleus contains a haploid number of chromosomes.

Animation Click on the link to see an animation of Meiosis: http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages%20of%20Meiosis